1. Field of the Invention
The invention relates to an air mass meter with a sensor element, wherein an air mass flow to be measured moves over the sensor element, and wherein the sensor element is embodied as a microelectromechanical system which has a diaphragm on which a heating element is embodied, wherein in each case one electrical measuring resistor and at least two electrical comparison resistors are arranged upstream and downstream of the heating element in the direction of the air mass flow, and wherein a first temperature sensor element and a second temperature sensor element are formed by the electrical connection of in each case one measuring resistor to at least two comparison resistors.
2. Related Art
Such air mass meters are used, for example, in motor vehicles for determining the air mass swept in by an internal combustion engine. Combustion can be optimized on the basis of the most reliable possible information about a sucked-in air mass by an electronic control of the internal combustion engine, to the effect that a quantity of fuel which is matched precisely to the air mass is fed to the respective combustion chamber. As a result, better utilization of energy with reduced emission of pollutants is achieved.
DE 44 07 209 A1 discloses an air mass meter plugged into an intake duct for determining an air mass, wherein a defined proportion of the total flow flows through the air mass sensor. For this purpose, the latter is embodied as a plug-in duct air mass meter. The air mass meter comprises a sensor element arranged in a measuring duct, electronics arranged in a housing for evaluating and/or for detecting the measured values of the sensor element, and an outlet duct on the other side of the sensor element. For a space-saving arrangement, the specified ducts or air-guiding paths are embodied in the form of a U, S or C, with the result that a device that is compact overall and is embodied as a plug-in element is formed.
US 2008/0282791 A1 discloses an air mass meter with a sensor element in which in each case temperature sensor elements are arranged upstream and downstream of a heating element embodied on a diaphragm. The temperature sensor elements are arranged in such a way that shifting of a characteristic curve owing to deposits of dirt is suppressed.
US 2003/0010110 A1 discloses a mass flow sensor that operates bidirectionally and has a bridge circuit connected to a voltage potential, wherein the bridge circuit contains a first and a second temperature-dependent sensor, which sensors are connected in series and are arranged on a thermally insulating substrate.
DE 42 08 135 A1 discloses a device for measuring a flow of gas or fluid. Here, two temperature-sensitive resistor devices form a first sensor, and two further temperature-sensitive resistor devices form a second sensor. The two sensors are arranged in such a way that they do not influence one another thermally.
An air mass meter embodied according to the teaching of WO 03/089884 A1 and embodied as a hot-film anemometer has in principle proven valuable.
During the development of modern air mass meters that operate on the basis of sensor elements embodied as microelectromechanical systems (MEMS) it has become apparent that the measurement results of the sensor elements are influenced in a particularly disadvantageous way by contamination. Contamination, which can be caused, for example, by oil droplets in the air mass flow, results over time in the occurrence of signal drift in the sensor element, which sensor drift can lead to incorrect measured values for the air mass flow. However, sensor elements embodied as microelectromechanical systems have a multiplicity of advantages which should not be dispensed with.